The invention relates to a measuring device and a method to optically measure an object.
Accordingly, the measuring device comprises an interferometer with a light source, with the interferometer being embodied such that a light beam created by the light source is split into at least two partial beams.
A first partial beam is used as a measuring beam for radiating a measuring point on the object to be measured. For this purpose the measuring beam essentially exits the interferometer at a predetermined angle. The measuring beam reflected by the object enters the interferometer as a reflection beam through a reflection beam entry and is here interfered with a second partial beam. The second partial beam and the interfering reflection beam exit the interferometer through an interference beam exit.
The measuring device is further provided with an optic detector, which is arranged at the interference beam exit of the interferometer such that the second partial beam impinges the detector together with the interfered reflection beam.
A signal processing unit of the measuring device is connected to the detector and measures the measuring signals of the detector. The measurement may comprise collection and/or evaluation and/or processing of the measuring signals.
In typical measuring devices of the above-described type the light source is embodied as a laser, which created an essentially monochromatic light beam. By interfering the second partial beam and the reflection beam an interference signal develops on the optic detector, from which for example the phase difference of the two interfered beams can be determined by way of demodulation, which in turn allows the determination of the displacement of the object in the area of the measuring point.
Furthermore, the actual speed of the movement of the measuring point of the object can be determined by demodulation of the temporal deduction of the phase difference.
The precision of the above-described measurements performed depends on several factors:
When in a grid scan, several points on the object to be measured are measured one after the other, the spatial resolution essentially depends on the size of the measuring spots, i.e. of the diameter of the measuring beam impinging the measuring point of the object to be measured.
If very small structures are measured, for example in micro technology, it is essential that the measuring spot has a maximum size equivalent to approximately the size of said micro structures. If a lateral resolution shall be achieved for the micro structures, for example via a grid scan, a further reduction of the size of the measuring spot is necessary. Typical micro structures require a diameter of the measuring spot smaller than 1 μm.
However, the precision of the measurement also depends on the quality of the signal measured by the optic detector. This signal, in turn, depends on the intensity of the reflection beam and thus also on the intensity, by which the object to be measured is impinged by the measuring beam.
In particular the measuring of oscillations of small structures in micro technology requires a measuring beam of high intensity in order to yield a good signal-noise ratio. This leads, together with the requirement of a small diameter of the measuring spot, to a high irradiance on the object to be measured.
Due to the high irradiance, the measurement may cause a high energy input into the object to be measured when conventional measuring devices are used for the optical measurement of an object, and thus lead to falsify the measurement and to the destruction of the object to be measured in particular in structures of micro technology.